Method and apparatus for dynamically allocating upstream bandwidth in passive optical networks

ABSTRACT

One embodiment of the present invention provides a system that facilitates dynamic allocation of upstream bandwidth in a passive optical network which includes a central node and at least one remote node. Each remote node is coupled to at least one logical entity, which corresponds to a device or a user, that transmits upstream data to the central node and receives downstream data from the central node. The central node is coupled to an external network outside of the passive optical network through a shared out-going uplink.

RELATED APPLICATIONS

This application is a continuation of and claims the benefit under 35U.S.C. section 120 of a U.S. patent application Ser. No. 10/663,608,filed 15 Sep. 2003, which has been allowed and is scheduled to issue on22 Apr. 2008 as U.S. Pat. No. 7,362,704.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the design of passive optical networks.More specifically, the present invention relates to a method andapparatus for dynamically allocating upstream bandwidth in a passiveoptical network.

2. Related Art

In order to keep pace with increasing Internet traffic, optical fibersand associated optical transmission equipment have been widely deployedto substantially increase the capacity of backbone networks. However,this increase in the capacity of backbone networks has not been matchedby a corresponding increase in the capacity of access networks. Evenwith broadband solutions, such as digital subscriber line (DSL) andcable modem (CM), the limited bandwidth offered by current accessnetworks creates a severe bottleneck in delivering high bandwidth to endusers.

Among different technologies, Ethernet passive optical networks (EPONs)appear to be the best candidate for next-generation access networks.EPONs combine the ubiquitous Ethernet technology with inexpensivepassive optics. Therefore, they offer the simplicity and scalability ofEthernet, and the cost-efficiency and high capacity of passive optics.In particular, due to the high bandwidth of optical fibers, EPONs arecapable of accommodating broadband voice, data, and video trafficsimultaneously. Such integrated service is difficult to provide with DSLor CM technology. Furthermore, EPONs are more suitable for InternetProtocol (IP) traffic, since Ethernet frames can directly encapsulatenative IP packets with different sizes, whereas ATM passive opticalnetworks (APONs) use fixed-size ATM cells and consequently requirepacket fragmentation and reassembly.

Typically, EPONs are used in the “first mile” of the network, whichprovides connectivity between the service provider's central offices andbusiness or residential subscribers. Logically, the first mile is apoint-to-multipoint network, with a central office servicing a number ofsubscribers. A tree topology can be used in an EPON, wherein one fibercouples the central office to a passive optical splitter, which dividesand distributes downstream optical signals to subscribers and combinesupstream optical signals from subscribers (see FIG. 1).

Transmissions within an EPON are typically performed between an opticalline terminal (OLT) and optical networks units (ONUs) (see FIG. 2). TheOLT generally resides in the central office and couples the opticalaccess network to the metro backbone, which is typically an externalnetwork belonging to an ISP or a local exchange carrier. The ONU can belocated either at the curb or at an end-user location, and can providebroadband voice, data, and video services.

Communications within an EPON can be divided into upstream traffic (fromONUs to OLT) and downstream traffic (from OLT to ONUs). Because of thebroadcast nature of Ethernet, the downstream traffic can be deliveredwith considerable simplicity in an EPON: packets are broadcast by theOLT and extracted by their destination ONU based on their media accesscontrol (MAC) addresses. However, in the upstream direction, the ONUsneed to share the channel capacity and resources. Moreover, theburstiness of network traffic and the requirement of different servicelevel agreements (SLAs) make the upstream bandwidth allocation achallenging problem.

Hence, what is needed is a method and apparatus for dynamicallyallocating upstream bandwidth in an EPON, which is fair, efficient, andresponsive, and which accommodates bursty traffic while satisfying SLAs.

SUMMARY

One embodiment of the present invention provides a system thatfacilitates dynamic allocation of upstream bandwidth in a passiveoptical network which includes a central node and at least one remotenode. Each remote node is coupled to at least one logical entity, whichcorresponds to a device or a user, that transmits upstream data to thecentral node and receives downstream data from the central node. Thecentral node is coupled to an external network outside of the passiveoptical network through a shared out-going uplink.

During operation, the system receives a request from a remote node for agrant to transmit upstream data from a logical entity associated withthe remote node to the central node, wherein the size of the data to betransmitted does not exceed a transmission threshold assigned to thatlogical entity, and a logical entity may not request more than what isallowed by the corresponding transmission threshold. If the requestsatisfies a bandwidth allocation policy, the system issues a grant tothe remote node to transmit upstream data. In response to the grant, thesystem receives upstream data from the remote node and places thereceived upstream data in a receiver buffer within the central node.This receiver buffer includes a number of FIFO queues, each of whichbuffers upstream data received from an associated logical entity. Next,the system retrieves and transmits data stored in the receiver buffer tothe out-going uplink according to a set of SLAs.

In a variation of this embodiment, satisfying the bandwidth allocationpolicy requires that there be sufficient available space in the receiverbuffer to accommodate the upstream data to be transmitted, and that thelogical entity from which upstream data transmission is requested isscheduled to transmit data next.

In a further variation, all the logical entities within the passiveoptical network are scheduled to transmit upstream data in ahierarchical round-robin scheme by performing the following operations:

(1) grouping logical entities with the highest priority to form atop-priority level;

(2) allowing each logical entity in the top-priority level to transmitupstream data in a round-robin fashion by assigning a slot to eachlogical entity in the top-priority level;

(3) within the top-priority level, reserving one slot for lower-prioritytraffic;

(4) grouping logical entities with the second-highest priority to form asecond-priority level;

(5) allowing each logical entity in the second-priority level totransmit data by assigning the reserved slot within the top-prioritylevel to each logical entity in the second-priority level in around-robin fashion;

(6) within the second-priority level, reserving one slot forlower-priority traffic; and

(7) repeating operations similar to operations (4)-(6) for logicalentities with lower priorities until every logical entity is assigned aslot for transmitting upstream data according to its priority.

In a variation of this embodiment, the transmission threshold assignedto a logical entity within a priority level is determined by consideringthe maximum allowable delay for that priority level, data speed of theshared out-going uplink, the logical entity's SLA, and the total numberof logical entities within that priority level.

In a variation of this embodiment, the system keeps a record ofoutstanding data for each logical entity, wherein outstanding data isupstream data for which a grant for transmission has been issued by thecentral node, but which has not been received by the central node. Tocalculate available space in the receiver buffer, the system subtractsthe size of outstanding data from the unfilled space of thecorresponding FIFO queue. After a period of time following issuance of agrant for transmitting a piece of data, the data is due to arrive at thesystem. The system accordingly removes the information pertinent to thepiece of data from the record of outstanding data for the correspondinglogical entity, which is done regardless of whether the piece of datahas actually been received by the central node.

In a variation of this embodiment, the system retrieves and transmitsdata stored in each FIFO queue within the receiver buffer in ahierarchical round-robin scheme in accordance with each logical entity'sSLA.

In a variation of this embodiment, each remote node includes a number ofqueues, each of which is associated with a logical entity and storesupstream data from the device or user associated with that logicalentity.

In a further variation, the request from the remote node reports thestate of a queue within that remote node associated with a logicalentity, and the request piggybacks on an upstream data transmission.

In a further variation, if a FIFO queue within the receiver buffer inthe central node is full, the system pauses the issuing of grants to thecorresponding logical entity, thereby causing the queue associated withthat logical entity within a remote node to become full. This causes theremote node to generate a flow-control message to the correspondingdevice or user to slow down or pause the upstream data transmission fromthat device or user.

In a variation of this embodiment, a remote node tracks the amount oftime between the grants to transmit upstream data for each logicalentity associated with the remote node. If the amount of time betweengrants exceeds a certain interval, the remote node sets an alarm andsends a message to the central node via an Operation, Administration andMaintenance (OAM) frame, whereby upon receiving the message, the centralnode is allowed to reset a record associated with the correspondinglogical entity.

In a variation of this embodiment, the central node periodically sendsout polls to the remote nodes to see if a logical entity has any data tosend. The polling frequency for a corresponding logical entity reflectsthe SLA of the logical entity. If a non-poll grant has been previouslysent to a logical entity, the subsequent poll to that logical entity issent at a time after the non-poll grant, the time being calculated inaccordance to the corresponding polling frequency.

In a further variation, a remote node tracks the amount of elapsed timebetween non-poll grants for each logical entity associated with theremote node. If the elapsed time between non-poll grants for a logicalentity exceeds a certain interval, the remote node sets an alarm. If thealarm is set and the remote node has data to send from the correspondinglogical entity, the remote node sends a message to the central node viaan OAM frame denoting an error condition, which instructs the centralnode that the logical entity is in an error state. Upon receiving themessage, the central node is allowed to reset or modify a recordassociated with the logical entity.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a passive optical network wherein a central officeand a number of subscribers form a tree topology through optical fibersand a passive optical splitter (prior art).

FIG. 2 illustrates a passive optical network including an OLT and ONUs(prior art).

FIG. 3 illustrates the architecture of an OLT that facilitates dynamicupstream bandwidth allocation in accordance with an embodiment of thepresent invention.

FIG. 4 presents a flow chart illustrating the dynamic upstream bandwidthallocation process in accordance with an embodiment of the presentinvention.

FIG. 5 illustrates a flow-control mechanism within an OLT thatfacilitates dynamic upstream bandwidth allocation in accordance with anembodiment of the present invention.

FIG. 6 illustrates a hierarchical round-robin scheduling scheme withtransmission thresholds in accordance with an embodiment of the presentinvention.

FIG. 7 illustrates a time-out mechanism for outstanding data thatprovides fault tolerance in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

The data structures and code described in this detailed description aretypically stored on a computer readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. This includes, but is not limited to, application specificintegrated circuits (ASICs), field-programmable gate arrays (FPGAs),semiconductor memories, magnetic and optical storage devices such asdisk drives, magnetic tape, CDs (compact discs) and DVDs (digitalversatile discs or digital video discs), and computer instructionsignals embodied in a transmission medium (with or without a carrierwave upon which the signals are modulated). For example, thetransmission medium may include a communications network, such as theInternet.

Passive Optical Network Topology

FIG. 1 illustrates a passive optical network, wherein a central officeand a number of subscribers form a tree topology through optical fibersand a passive optical splitter. As shown in FIG. 1, a number ofsubscribers are coupled to a central office 101 through optical fibersand a passive optical splitter 102. Passive optical splitter 102 can beplaced in the vicinity of end-user locations, so that the initial fiberdeployment cost is minimized. The central office is coupled to anexternal network, such as a metropolitan area network operated by anISP.

FIG. 2 illustrates a passive optical network including an OLT and ONUs.OLT 201 is coupled with ONUs 202, 203, and 204 through optical fibersand a passive optical splitter. An ONU can accommodate a number ofnetworked devices, such as personal computers, telephones, videoequipment, network servers, etc. Note that a networked device canidentify itself by using a Logical Link ID (LLID), as defined in theIEEE 802.3 standard.

Dynamic Bandwidth Allocation Mechanism

FIG. 3 illustrates the architecture of an OLT that facilitates dynamicupstream bandwidth allocation in accordance with an embodiment of thepresent invention. In this example, an OLT 320 accepts requests andupstream data traffic from ONUs 301 and 302. Each ONU maintains a numberof queues, for example queues 311, 312, and 313, each of which storesupstream data from an LLID corresponding to a device or a user thatcouples to that ONU. Note that upstream data from an LLID is carried indata frames (e.g., Ethernet frames), which have variable sizes. Duringtransmission these data frames are removed from their respective queue.An LLID requests a grant, to transmit upstream data, via a reportmessage. The report message indicates the amount of data in the LLID'scorresponding queue(s). Typically, these request messages can piggybackon an upstream data transmission.

Within OLT 310, a dynamic bandwidth allocation (DBA) scheduler 303receives the report messages from ONUs. OLT 310 also includes a FIFOqueue controller (FCT) 305, which contains a number of FIFO queues (321,322, 323, 324, and 325) that are associated with different LLIDs.Upstream data from each LLID is temporarily stored in these FIFO queuesbefore being transmitted to the external ISP network through a shareduplink 330. The state of these FIFO queues is monitored and stored in aqueue length table 304.

After receiving a request from an LLID, DBA scheduler 303 determineswhether a grant to transmit can be sent to the requesting LLID based ontwo considerations. First, whether there is sufficient available spacein the FIFO queue corresponding to the requesting LLID, according queuelength table 304. Second, whether the requesting LLID is the next inturn to transmit data as scheduled. (Note that proper scheduling ofLLIDs for upstream data transmission is necessary to guarantee fair andefficient bandwidth allocation among all the LLIDs.) When bothconditions are met, the DBA scheduler issues a grant to the requestingLLID. The grant allocates an upstream transmission time slot to theLLID.

Note that outstanding data for each LLID can be taken into account inthe calculation of available space in the FIFO queues. Outstanding datais the “in-flight” data for which a grant for transmission has beengiven, but which has not been received by OLT 320. Records ofoutstanding data are stored in data structure 309. When calculatingavailable space in a FIFO queue, DBA scheduler 303 subtracts the amountof outstanding data of the requesting LLID from the available physicalspace in the corresponding FIFO queue, and uses the result as the actualavailable space for future data transmission.

With regard to scheduling upstream transmission, one possible scheme isthe hierarchical round-robin scheme, which can be used to fairly andefficiently allocate bandwidth among all LLIDs. Another possiblescheduling scheme is strict priority scheduling. However, because SLAsusually place constraints on parameters such as average bit rate,maximum delay, etc., a transmission threshold (the maximum amount ofdata in each transmission) may be set for every LLID in the hierarchicalround-robin scheme. A more detailed discussion of this scheme appears inthe discussion related to FIG. 5 below.

OLT 320 further includes a bandwidth shaper 307, which retrieves datastored in the FIFO queues within FCT 305 and transmits the retrieveddata to shared uplink 330. Bandwidth shaper 307 ensures that the datastored in FCT 305 is served in accordance with the priorityclassification and SLA pertinent to each LLID, which is stored in datastructure 306. Like the scheduling mechanism within DBA scheduler 303,the scheduling mechanism within bandwidth shaper 307 is desired to befair and efficient, and therefore can also use the hierarchicalround-robin scheduling scheme.

FIG. 4 presents a flow chart illustrating the dynamic upstream bandwidthallocation process in accordance with an embodiment of the presentinvention. The system starts by receiving a report message from an LLIDat the DBA scheduler 303 (step 401). DBA scheduler 303 then determinesif there is sufficient space in the FIFO queue within FCT 305 for thisLLID (taking into account the outstanding data) (step 402). If there isnot sufficient space, DBA scheduler temporarily holds the grant for therequesting LLID until sufficient space becomes available in the FIFOqueue. Meanwhile, the system can receive and process requests from otherLLIDs by returning to step 401.

If there is sufficient space in the FIFO queue within FCT 305, DBAscheduler 303 further determines if the requesting LLID is scheduled totransmit data next (step 403). If not, DBA scheduler 303 willtemporarily hold the grant until the requesting LLID is the next totransmit. Meanwhile, the system can receive and process requests fromother LLIDs by returning to step 401.

If it is the requesting LLID's turn to transmit, DBA scheduler generatesa grant and sends it to the requesting LLID (step 404). The system thenreturns to step 401 and continues to receive and process subsequentrequests.

Flow-Control Mechanism

FIG. 5 illustrates a flow-control mechanism within an OLT thatfacilitates dynamic upstream bandwidth allocation in accordance with anembodiment of the present invention. In this example, when FIFO queue323 is filled, DBA scheduler 303 stops granting transmission from LLID#3, thereby causing queue 313 to fill. ONU 302 can then generate aflow-control message in accordance with the IEEE 802.3x standard to thecorresponding device or user to slow down, or pause, further upstreamdata transmission.

Hierarchical Round-Robin Scheduling with Transmission Thresholds

FIG. 6 illustrates a hierarchical round-robin scheduling scheme withtransmission thresholds in accordance with an embodiment of the presentinvention. This hierarchical round-robin scheduling is performed asfollows:

First, group all LLIDs with the highest priority (priority 0). Withinpriority 0, assign each LLID a transmission slot in accordance to anamount of data burst the LLID is allowed to transmit upstream. The LLIDis provisioned to not report a value greater than this amount. Althoughthe aggregate of all report messages in a report frame may exceed thisthreshold, the amount of data implied in each individual message cannotexceed this burst size. The slot size provisioned for each LLID isdetermined such that all the LLIDs may be serviced within a fixed delaybounds. For example, if the delay bounds for priority 0 is one ms, andshared uplink 330's data speed is 1 Gb/s, then the total duration ofpriority 0 may not exceed 1000 Kb. Therefore, the aggregate slot size ofpriority 0 LLIDs would sum up to less than or equal to 1000 Kb.

Within priority 0, one slot is allocated for lower priority traffic.This slot is denoted as the drop-down slot. All lower-priority trafficis allowed to transmit within this reserved slot.

Next, group all of the LLIDs with the second highest priority (priority1). Within priority 1, assign each LLID a transmission slot according tothe maximum burst the LLID may transmit upstream. The LLID will beconfigured such that it will observe this maximum burst size whenreporting. A slot in priority 1 is allowed to transmit inside the slotreserved for lower-priority traffic (the drop-down slot) within priority0. Since a priority 1 LLID may only transmit when priority 0 istransmitting its drop-down slot, the delay of the queuing delay ofpriority 1 LLIDs is typically many times of the queuing delay ofpriority 0 LLIDs.

Within priority 1, there is similarly one slot reserved forlower-priority traffic.

As shown in FIG. 6, one can repeat steps similar to the above, andconstruct an entire hierarchy to accommodate all the LLIDs. Note thatthe transmission thresholds of LLIDs within a given priority level isbased on the bandwidth and maximum allowable delay negotiated in thecorresponding SLA.

Fault Tolerance

FIG. 7 illustrates a time-out mechanism for outstanding data thatprovides fault tolerance in accordance with an embodiment of the presentinvention. During operation, it is possible that a grant message 731 islost on its way from OLT 720 to ONU 610, for example due to a bit error.As a result, the subsequent grant messages received by ONU 710 for thesame LLID will grant transmission sizes that are inconsistent with theamount of data available for upstream transmission. This may manifestitself by the ONU receiving a grant that is not a frame boundary. OnceONU 710 detects this inconsistency, it will start sending special reportmessages to OLT 720, requesting a transmission size of 0 Kb. Meanwhile,OLT 720 keeps track of when a piece of upstream data associated with agrant is due to arrive. Whether or not this piece of data physicallyarrives for the grant, the OLT removes the information corresponding tothe outstanding data for the grant.

After sending the special report messages (with request of 0 K) for aperiod of time, ONU 710 resumes sending normal request messages. By thistime the lost grant message, and its residual effects, would have timedout in OLT 720 and normal operation resumes.

It is possible for an ONU to track the amount of time between grants. Ifthe amount of time between grants exceeds a certain interval, ONU 710sets an alarm and sends a message to OLT 720 via an OAM frame. This canbe done via an LLID on the ONU that is reserved for processor traffic.This message will instruct OLT 720 that an LLID is not being granted.One way for OLT 720 to deal with this situation is to reset the LLIDentry in the DBA and bandwidth shaper tables.

In another scenario, OLT 720 periodically sends out polls to ONUs to seeif an LLID has any data to send. Polls are grants for 64 bytes of datathat have a forced-report flag asserted. The only upstream datatransmitted as a response to a poll is a single report frame. Thepolling frequency reflects the SLA of an LLID. For example, the pollsfor priority 0 LLIDs are sent every 1 ms. If a grant previouslyoccurred, the subsequent poll will be sent at 1 ms after that grantbeing sent.

Correspondingly, a non-poll grant is a grant that allows transmission ofmore than just a single report frame. An ONU tracks the amount of timeelapsed between non-poll grants for each LLID. If this time exceeds acertain interval, the ONU sets an alarm. If the alarm is set, and theONU has data to send, the ONU will send a message to the OLT, via an OAMframe, denoting the error condition. This will instruct the OLT that anLLID is in an error state. One way for the OLT to deal with thissituation is to reset or modify the LLID entry in the DBA and bandwidthscheduler tables.

The foregoing descriptions of embodiments of the present invention havebeen presented for purposes of illustration and description only. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

1. A method for dynamically allocating upstream bandwidth in a passive optical network, the method comprising: determining at an Optical Line Terminal (OLT) an upstream transmission threshold dynamically for a logical entity associated with an Optical Network Unit (ONU) based on one or more of the following: the maximum allowable delay associated with a priority level assigned to the logical entity; data speed of a shared out-going uplink at the OLT; the logical entity's service level agreement; and the total number of logical entities within that priority level; receiving a request from the ONU to transmit upstream data from the logical entity, wherein the logical entity may not request to transmit more data than what is allowed by the transmission threshold; issuing a grant to the ONU for transmitting upstream data; in response to the grant, receiving upstream data from the ONU; storing the received upstream data in a receiver buffer within the OLT; keeping a record of outstanding upstream data which is granted for transmission from the ONU but not yet received by the OLT for each logical entity; subtracting the size of outstanding upstream data from the unfilled space of the receiver buffer to calculate available space in the receiver buffer; and transmitting the received upstream data on the shared out-going uplink.
 2. The method of claim 1, wherein the receiver buffer includes a number of FIFO queues, each of which buffers upstream data received from an associated logical entity; and wherein transmitting the received upstream data involves retrieving and transmitting the upstream data stored in the receiver buffer to the out-going uplink according to a set of service level agreements.
 3. The method of claim 2, further comprising: determining whether the receiver buffer has sufficient space to accommodate the upstream data to be transmitted as requested; and determining whether the logical entity from which upstream data transmission is requested is scheduled to transmit data next.
 4. The method of claim 1, further comprising scheduling a number of logical entities within the passive optical network to transmit upstream data using a hierarchical round-robin scheme.
 5. The method of claim 4, wherein scheduling the logical entities to transmit upstream data comprises: (1) grouping logical entities with the highest priority to form a top-priority level; (2) allowing each logical entity in the top-priority level to transmit upstream data in a round-robin fashion by assigning a slot to each logical entity in the top-priority level; (3) within the top-priority level, reserving at least one slot for lower-priority traffic; (4) grouping logical entities with the next-highest priority to form a next-highest-priority level; (5) allowing each logical entity in the next-highest-priority level to transmit data by assigning the reserved slot within the immediately higher-priority level to each logical entity in the next-highest-priority level in a round-robin fashion; (6) within the next-highest-priority level, reserving at least one slot for lower-priority traffic; and (7) repeating operations similar to operations (4)-(6) for logical entities with lower priorities until every logical entity is assigned a slot for transmitting upstream data according to its priority.
 6. The method of claim 1, wherein all the logical entities within the passive optical network are scheduled to transmit upstream data using a strict priority scheduling scheme.
 7. The method of claim 1, further comprising: timing out the outstanding upstream data after a predetermined period.
 8. The method of claim 2, wherein retrieving and transmitting data stored in the receiver buffer to the out-going uplink according to a set of service level agreements involves retrieving and transmitting data stored in each FIFO queue using a hierarchical round-robin scheme in accordance with each logical entity's service level agreement.
 9. The method of claim 1, wherein a respective ONU includes a number of queues, each of which is associated with a logical entity and stores upstream data from that logical entity.
 10. The method of claim 9, wherein the request from an ONU reports the state of a queue within that remote node associated with a logical entity; and wherein the request piggybacks on an upstream data transmission.
 11. The method of claim 9, wherein if a FIFO queue within the receiver buffer in the OLT is full, the method further comprises pausing the issuance of grants to the corresponding logical entity, thereby causing the queue associated with that logical entity within the corresponding ONU to become full, upon which the ONU generates a flow-control message to the corresponding logical entity to slow down the upstream data transmission from that device or user.
 12. The method of claim 1, further comprising: periodically sending polls to the ONU for upstream data; wherein the polling frequency for a corresponding logical entity reflects the service level agreement of that logical entity.
 13. A system for dynamically allocating upstream bandwidth in a passive optical network, the system comprising: a bandwidth allocation mechanism configured to determine at an Optical Line Terminal (OLT) an upstream transmission threshold dynamically for a logical entity associated with an Optical Network Unit (ONU) based on one or more of the following: the maximum allowable delay associated with a priority level assigned to the logical entity; data speed of a shared out-going uplink at the OLT; the logical entity's service level agreement; and the total number of logical entities within that priority level; a request receiving mechanism configured to receive a request from the ONU to transmit upstream data from the logical entity, wherein the logical entity may not request to transmit more data than what is allowed by the transmission threshold; a grant issuance mechanism configured to issue a grant to the ONU for transmitting upstream data; a data receiving mechanism configured to receive upstream data from the ONU; a storage mechanism configured to store the received upstream data in a receiver buffer within the OLT; a record-keeping mechanism configured to keep a record of outstanding upstream data which is granted for transmission from the ONU but not yet received by the OLT for each logical entity; a subtraction mechanism configured to subtract the size of outstanding upstream data from the unfilled space of the receiver buffer to calculate available space in the receiver buffer; and a transmission mechanism configured to transmit the received upstream data on the shared out-going uplink.
 14. The system of claim 13, wherein the receiver buffer includes a number of FIFO queues, each of which buffers upstream data received from an associated logical entity; and wherein while transmitting the received upstream data, the transmission mechanism is configured to retrieve and transmit the upstream data stored in the receiver buffer to the out-going uplink according to a set of service level agreements.
 15. The system of claim 14, wherein the storage mechanism is further configured to: determine whether the receiver buffer has sufficient space to accommodate the upstream data to be transmitted as requested; and determine whether the logical entity from which upstream data transmission is requested is scheduled to transmit data next.
 16. The system of claim 13, further comprising a scheduling mechanism configured to schedule a number of logical entities within the passive optical network to transmit upstream data using a hierarchical round-robin scheme.
 17. The system of claim 16, wherein while scheduling the logical entities to transmit upstream data, the scheduling mechanism is configured to: (1) group logical entities with the highest priority to form a top-priority level; (2) allow each logical entity in the top-priority level to transmit upstream data in a round-robin fashion by assigning a slot to each logical entity in the top-priority level; (3) within the top-priority level, reserve at least one slot for lower-priority traffic; (4) group logical entities with the next-highest priority to form a next-highest-priority level; (5) allow each logical entity in the next-highest-priority level to transmit data by assigning the reserved slot within the immediately higher-priority level to each logical entity in the next-highest-priority level in a round-robin fashion; (6) within the next-highest-priority level, reserve at least one slot for lower-priority traffic; and (7) repeat operations similar to operations (4)-(6) for logical entities with lower priorities until every logical entity is assigned a slot for transmitting upstream data according to its priority.
 18. The system of claim 13, wherein all the logical entities within the passive optical network are scheduled to transmit upstream data using a strict priority scheduling scheme.
 19. The system of claim 14, further comprising: a time-out mechanism configured to time out the outstanding upstream data after a predetermined period.
 20. The system of claim 14, wherein while retrieving and transmitting data stored in the receiver buffer to the out-going uplink according to a set of service level agreements, the transmission mechanism is configured to retrieve and transmit data stored in each FIFO queue using a hierarchical round-robin scheme in accordance with each logical entity's service level agreement.
 21. The system of claim 13, wherein a respective ONU includes a number of queues, each of which is associated with a logical entity and stores upstream data from that logical entity.
 22. The system of claim 21, wherein the request from an ONU reports the state of a queue within that ONU associated with a logical entity; and wherein the request piggybacks on an upstream data transmission.
 23. The system of claim 21, wherein if a FIFO queue within the receiver buffer in the OLT is full, the grant issuance mechanism is further configured to pause the issuance of grants to the corresponding logical entity, thereby causing the queue associated with that logical entity within the corresponding ONU to become full, upon which the ONU generates a flow-control message to the corresponding logical entity to slow down the upstream data transmission from that device or user.
 24. The system of claim 13, further comprising: a polling mechanism configured to periodically send polls to the ONU for upstream data; wherein the polling frequency for a corresponding logical entity reflects the service level agreement of that logical entity.
 25. An Optical Line Terminal (OLT) for dynamically allocating upstream bandwidth in a passive optical network, the OLT comprising: a bandwidth allocator configured to determine dynamically an upstream transmission threshold for a logical entity associated with an Optical Network Unit (ONU) based on one or more of the following: the maximum allowable delay associated with a priority level assigned to the logical entity; data speed of a shared out-going uplink at the OLT; the logical entity's service level agreement; and the total number of logical entities within that priority level; a request receiving mechanism configured to receive a request from the ONU to transmit upstream data from the logical entity, wherein the logical entity may not request to transmit more data than what is allowed by the transmission threshold; a grant issuing mechanism configured to issue a grant to the ONU for transmitting upstream data; a data receiving mechanism configured to receive upstream data from the ONU; a storage mechanism configured to store the received upstream data in a receiver buffer within the OLT; a record-keeping mechanism configured to keep a record of outstanding upstream data which is granted for transmission from the ONU but not yet received by the OLT for each logical entity; a subtraction mechanism configured to subtract the size of outstanding upstream data from the unfilled space of the receiver buffer to calculate available space in the receiver buffer; and a transmission mechanism configured to transmit the received upstream data on the shared out-going uplink. 